This invention relates to magnetostrictive surface acoustic wave (SAW) devices and, in particular, to magnetostrictive SAW devices with transducers tuned for optimal magnetic anisotropy and to methods for making such devices.
SAW devices are important components in RF circuits, especially in wireless communication devices. SAW devices are particularly important as delay elements. They provide low-velocity, non-dispersive propagation with low attenuation up to microwave frequencies and a propagation path which is accessible at a substrate surface.
While conventional SAW devices are not readily integrated into silicon microelectronic circuits, a magnetostrictive SAW device which can be fabricated on silicon substrates is described in U.S. patent application Ser. No. 09/137,431, filed by Glenn B. Alers et al. on Aug. 21, 1998 and entitled xe2x80x9cMagnetostrictive Acoustic Wave Device and Microelectronic Circuit Including Samexe2x80x9d, which is incorporated herein by reference. These devices can be integrated with microelectronic circuits usefull in wireless communications.
In essence, the magnetostrictive SAW device comprises a substrate, a film of magnetostrictive material disposed on the substrate, an input transducer for generating surface acoustic waves along the film and an output transducer for receiving the acoustic waves. The substrate can comprise silicon and include one or more microelectronic circuit elements interconnected with the magnetostrictive SAW device. The transducers are typically serpentine (meander-type) electrodes.
Magnetostrictive SAW devices with improved transducer structures are described in U.S. patent application Ser. No. 09/464,805 filed concurrently herewith by Glenn B. Alers et al. The application, entitled xe2x80x9cMagnetostrictive Surface Acoustic Wave Devices Having Improved Transducersxe2x80x9d is incorporated herein by reference and describes, inter alia, improved transducer structures utilizing multiple configured layers. Typically the transducer conductors are disposed on the magnetostrictive film and covered with an overlying film of magnetic material. The overlying film reduces the magnetic reluctance of the magnetic circuit surrounding the conductor, thereby enhancing the magnetostrictive response of the device.
SAW devices exhibit two types of magnetic anisotropy: an intrinsic anisotropy of the magnetostrictive material and a shape anisotropy due to the geometry of the magnetic layers in the device. These combine to form the total anisotropy of the device. A certain amount of anistropy is needed to place the ferromagnetic resonance frequency above the devices operating frequency. This can be achieved through shape or intrinsic anistropy or both. While a certain level of total anisotropy is needed to achieve the device operating frequency, the higher the total anistropy, the larger the device must be. Accordingly there is a need for SAW devices tuned to the minimum total magnetic anisotropy consistent with their operating frequency.
In accordance with the invention, magnetostrictive SAW devices are provided with tunable transducer structures for enhanced performance. The transducers are in the form of patterned conductors disposed between magnetic films. The magnetostrictive response is tuned by adjusting the thickness of magnetic film overlying the conductive layer.